Why pH is NOT an Acceptable Concentration Control Method

Q: Why does corrosion occur at too low and too high a concentration?
A: A too-low concentration has less inhibitor package, and a too-high concentration has too much activator package. Concentration within the setting range is important.
Q: Which is more accurate for concentration monitoring, RI or SV?
A: SV is more accurate due to less interference from flux or other soluble contaminants in the bath, but if the process window of concentration is wide enough, RI is still good and helpful.
Q: If white residue (insufficient clean) occurs due to a too-low concentration, could it be removed by re-cleaning with a higher concentration?
A: Normally, it requires strong mechanical energy to remove white residue. If the white residue is at a place like under small standoff gaps or between fine pitches, it would be hard to remove it, so dialing in the process to leave no residue with process control is the best way to remove white residue.